Metamorphosis in the Cirripedia Rhizocephala and the homology of the kentrogon and trichogon

The Rhizocephala are considered to be monophyletic due to several synapomorphies in the ontogeny of the cndoparasitic phase. The various types of metamorphosis described in the Rhizocephala are discussed and compared to metamorphosis in the Cirripedia Thoracica and Acrothoracica. In males and females of the suborder Kentrogonida. the cyprid settles and metamorphoses into a new instar, in males the trichogen and in females the infective kentrogon. The kentrogon goes through yet another. incomplete moult associated with the development of the stylet. Within the three kentrogonidan families. the Iernaeodiscid-peltogastrid type of kentrogon differs from the sacculinid type in the mode of attachment to the host. in the complexity of internal anatomy. in the position and penetration of the stylet, and in whether or not the cyprid carapace must be shed prior to penetration of the stylet. In the Akentrogonida metamorphosis never results in a new instar. Where observed (Clistosaccidae and Thompsoniidae). both male and female cyprids settle and penetrate into their substrate (female parasite or new host) with one of the antennules. Using the antennule as a syringe. male cyprids inject spermatogonia while female cyprids injects embryonic cells developing into an endoparasite. By comparison with metamorphosis in the Cirripedia Thoracica and Acrothoracica it is concluded that the presence of a metamorphic moult leading to a post-cyprid instar is plesiomorphic and that the trichogon and kentrogon are homologous with the first metamorphosed juvenile in these outgroups. The abbreviated ontogeny in the Akentrogonida without metamorphic moult and post-cyprid larval instars is considered apomorphic. This contradicts the long-held supposition that the Akentrogonida are the most‘primitive’Rhizocephala and dovetails with new information that this suborder contains many advanced traits. Within the Kentrogonida. the lernacodiseid-peltogastrid type of kentrogon is considered more plesiomorphic than the sacculinid type, which resembles the clistosaccidthompsoniid type in having the antennules involved in the penetration process. The homologization of the kentrogon with a juvenile barnacle indicates that presence of a kentrogon is plesiomorphic within the Rhizocephala and that the Kentrogonida is paraphyletic.     

The cypris larvae of the parasitic barnacle Heterosaccus lunatus (Crustacea, Cirripedia, Rhizocephala): some laboratory observations

Heterosaccus lunatus parasitizes the portunid crab, Charybdis callianassa in Moreton Bay, Australia. With the host crabs maintained at 22.5 degrees C this sacculinid rhizocephalan released larval broods every 6-7 days. During July-August 1996 and particularly August 1999 such broods showed the change-over from male only larvae in the early broods to females only in the later broods. As the host crabs were maintained under similar aquarium conditions in both years it is concluded that the light/dark cycle is the principal cue triggering this larval sex reversal. Oogenesis in the parasite externa is somehow controlled to produce two different sized ova - male larvae develop from large ova and females from small ova. A working hypothesis outlining how sex is probably determined for the larvae of sacculinids is erected. H. lunatus is considered the ideal sacculinid for the further experimental work necessary to verify the proposed sex-determining mechanism and its control processes. Measurements of the maximum swimming speeds of H. lunatus male and female cyprids showed the larger males to be the faster in absolute terms (27.95 compared with 17.60 mm s(-1), respectively), however, the calculated relative speeds were almost identical at approximately 90 body lengths s(-1). Settlement experiments confirmed that female H. lunatus cyprids settle only on the gills of C. callianassa; these cyprids needed to be at least 2 days old before they were able to settle.     

Cypris Ultrastructure,Metamorphosis and Sex in Seven Families of Parasitic Barnacles (Crustacea: Cirripedia: Rhizocephala)

Cypris morphology in the Rhizocephala differs both between the sexes of individual species and between the species. The large aesthetasc on the third antennular segment is unique to male cyprids of the Lernaeodiscidae, Peltogasteridae and Sacculinidae (suborder Kentrogonida) and Mycetomorpha vancouverensis (suborder Akentrogonida). A fourth segmental aesthetasc, common to both sexes, is much longer in males than in females. A spinous process on the attachment disc is another male-specific character, but is absent altogether in the Lernaeodiscidae. It is argued that these aesthetascs help locate the settlement targets. Except in Mycetomorpha vancourverensis, aesthetascs have been secondarily lost in cyprids of the suborder Akentrogonida, and there are no other morphological means for separating the male and female settling stage. In cyprids of Sylon hippolytes (Sylonidae) and of Mycetomorpha vancouverensis and Thompsonia sp. (Akentrogonidae) the long and slender antennules are probably used in penetration without formation of a kentrogon, as already reported for Clistosaccus paguri (Clistosaccidae). Cyprids of Sylon hippolytes and Clistosaccus paguri are almost morphologically identical.     

Antennulary sensory organs in cyprids of Octolasmis and Lepas (Crustacea: Thecostraca: Cirripedia: Thoracica): a scanning electron microscopic study

Cypris larvae of the pedunculate barnacles Octolasmis angulata (Poecilasmatidae), Lepas australis, L. pectinata, and Dosima fascicularis (Lepadidae) were studied with scanning electron microscopy, focusing on the sensory setae and the attachment disc on the antennules. The antennules of O. angulata did not exhibit any remarkable trait, but carry the same number of setae as seen in most other thoracicans. The third segment is bell-shaped and quite distinct from the second and its attachment disc is surrounded by a skirt. We found several potential synapomorphies in antennulary morphology between cyprids of the lepadid species but none of them were shared with the cyprids of Octolasmis; the list of unique lepadid characters includes: one additional, preaxial seta on the second segment; multiple similar (up to eight) postaxial setae (PS3) on the third segment, unlike all other thoracicans, where there is only a single PS3; the third segment consists almost entirely of the attachment disc, which is distended and surrounded by two parallel rows of radial setae; on the fourth segment the terminal seta E is diminutive. We found no traits in cyprids of Octolasmis that seem to be adaptations to their attachment site within the branchial chamber of swimming crabs and, in particular, no similarities with cyprids of rhizocephalan barnacles, many of which also attach in the gill chamber. The synapomorphies between cyprids of the lepadid species may be adaptations to their life in the neuston.     

On the morphology of antennular sensory and attachment organs in cypris larvae of the deep‐sea vent/seep barnacles,Ashinkailepas and Neoverruca

Barnacle cypris larvae show high morphological variation in the organs used in search of and attaching to a substratum. This variation may represent adaptation to the habitat of the species. Here, we studied SEM level morphologies of cypris antennular sensory and attachment organs in a deep‐sea vent endemic species (Neoverruca sp.) and a vent/seep inhabiting species (Ashinkailepas seepiophila). We compare them with three species from other environments. The antennular morphologies of Neoverruca sp. and A. seepiophila were similar, which is consistent with recent molecular studies showing a close relationship of the two species. The setation pattern of the antennules was very conservative among species from various environments. In contrast, striking differences were observed in the structure of the attachment organ (the third antennular segment). Neoverruca sp. and A. seepiophila had no velum or a skirt surrounding the attachment disc on the third segment, while other cirripede cyprids almost always have either of these structures. In addition, both cyprids of A. seepiophila and Neoverruca sp. had the attachment disc angled toward the substratum, whereas it faces distally in cyprids from hard bottom inhabiting barnacles. We suggest that both velum/skirt and the angle of the attachment disc play an important role, when the antennules are contacting the substratum during surface exploration. Differences in attachment organ structures may be highly adaptive, enabling cirripede species to enter new habitats during evolution. J. Morphol. 277:594–602, 2016. © 2016 Wiley Periodicals, Inc.     

The anatomy and development of the rhizocephalan barnacle Clistosaccus paguri Lilljeborg and relation to its host pagurus bernhardus (L.)

The anatomy of all developmental stages of Clistosaccus paguri Lilljeborg including internal primordia only 200 μm across has been studied by conventional and ultrastructural techniques. The earliest primordia lack any roots and organs. A receptacle and ovary arise as separate structures. The colleteric gland develops by infolding of the mantle cavity epithelium. The primordium becomes external during a host intermoult. The externa is apparently female. It is proposed that male cyprids, through the integument, have implanted cells found in the mantle of the externa. These male cells travel through the mantle into the single receptacle, where they proceed with spermatogenesis. Externae lacking male cells will not grow. The larvae are released as cyprids, in which no preformed kentrogon is present. About half of the infected hosts carry more than one externa. Multiple externae are separate and may represent different invading female cyprids. C. paguri infects all sizes of its host, and castrates it. The effect of the parasite on the host is discussed. The life span of the parasite seems to be one year and the parasite population shows an annual cycle.     

How whale and dolphin barnacles attach to their hosts and the paradox of remarkably versatile attachment structures in cypris larvae

How larvae of whale and dolphin epibionts settle on their fast-swimming and migrating hosts is a puzzling question in zoology. We successfully reared the larvae of the whale and dolphin barnacle Xenobalanus globicipitis to the cyprid stage. We studied the larval developmental ecology and antennular morphology in an attempt to assess whether an epibiotic lifestyle on this extreme substratum entails any unique larval specializations. Morphological parameters were compared with five other barnacle species that also inhabit extreme substrata. We found no larval specializations to a lifestyle associated with marine mammals. The external morphology of the antennules in Xenobalanus cyprids is morphologically similar to species from strikingly different substrata. We found variation only in the structures that are in physical contact with the substratum, i.e., the third segments carrying the villi-covered attachment disc. The third segments of the Xenobalanus cyprid antennules are not spear-shaped as in the stony coral barnacles, which are here used to penetrate the live tissue of their hosts. The presence of a cyprid cement gland implies that Xenobalanus uses cement protein when attaching to its cetacean host. Naupliar instars developed outside of the mantle cavity, indicating dispersal is planktonic. Our results militate against the idea that the cyprids settle during ocean migrations of their hosts. We suggest cyprids settle during coastal aggregations of the cetacean hosts. We conclude that the ecological success of barnacles has ultimately depended on a larva that with little structural alteration possesses the ability to settle on an amazingly wide array of substrata, including cetaceans.

     

Cypris Settlement,Kentrogon Formation and Host Invasion in the Parasitic Barnacle Lernaeodiscus porcellanae (Müller) (Crustacea: Cirripedia: Rhizocephala)

The anatomy of the female cypris is very similar to that of cyprids of the Thoracica. There is no development of a preformed kentrogon within the unsettled cypris. Kentrogon formation is completed within 10–20 min after settlement. Elimination of the spent cypris is necessary for successful development. The kentrogon includes a variety of tissues, such as muscles and part of the cement glands. The injection stylet develops in connection with a moult, and it penetrates into the crab gill through the ventral body wall of the kentrogon c. 50 h after settlement. Only a single cell, also recognizable in the unsettled cypris, is injected into the host, while the rest of the kentrogon degenerates after invasion. The speed of kentrogen development and much of the kentrogon anatomy can be interpreted as a defense against host crab grooming. The single invasion cell may either be considered as a stem cell or an egg.     

The relation between cypris ultrastructure and metamorphosis in male and female Sacculina carcini (Crustacea,Cirripedia)

Summary To elucidate current controversies on sex in rhizocephalan barnacles, broods of Sacculina carcini, infesting the shore crab Carcinus maenas, were raised to cyprids in the laboratory and followed through settlement and metamorphosis. Free-swimming cyprids were studied by transmission electron microscopy and occur in male and female morphological types, which differ in the structure of carapace cuticle, antennular cuticle, antennular glands, and the cells suspected of being the stem cells during metamorphosis. These dissimilarities are in addition to the already known differences in cypris size, in number of antennular sense organs, and in substrata settled on by morphological males and females. Metamorphosing males (trichogons) and females (kentrogons) are illustrated in interference phase-contrast micrographs. The morphological differences between male and female cyprids are directly related to their dissimilar metamorphosis. Hence, cyprids of male morphology are anatomically incapable of metamorphosing into kentrogons, while cyprids of female morphology cannot metamorphose into trichogons. The determination of sex in rhizocephalan barnacles is discussed.The results refute the hypothesis that sex in Sacculina carcini is determined environmentally, e.g., by the substratum encountered by the cyprids at settlement. It is concluded that sex is determined already in the free-swimming larvae and, most probably, already in the ovary. This agrees with the mode of sex determination in other species of the Rhizocephala Kentrogonida.     

Development of functional adaptation to clasping behaviour in harpacticoid copepods (Copepoda,Harpacticoida)

The copepodite antennule development of several harpacticoid families is studied and male antennule development of 5 species is schematically illustrated. The origin of newly formed segments can be determined by seta numbers of segments, relative segment length, furrows indicating previous articulations and by the position of the segment which bears the proximal aesthetasc. At least in some species sexual dimorphism of antennules is present from C II onwards. Females practically reach the adult antennular state at C V whereas males undergo drastic changes from C V to C VI with proximal addition of segments, and often with distal fusions and formation of a unique armature due to functional adaptations in clasping behaviour. No correlation has been found between clasping mode, male antennule type and taxonomic level. In closely related species of the same genus both chirocer and subchirocer antennules are found. The postnaupliar development of the chirocer type can proceed along different ways. Consequently it is suggested that at least chirocer antennules have originated several times independently during harpacticoid evolution.     

Cypris larvae of acrothoracican barnacles (Thecostraca: Cirripedia: Acrothoracica)

We used SEM to investigate the morphology of the cypris larvae from a range of species of the Cirripedia Acrothoracica, representing all three families and including the first detailed account of cyprids in the highly specialized Cryptophialidae. Special attention was given to the head shield (carapace), the lattice organs, the antennules, the thoracopods, the telson and the furcal rami. The cypris larvae of the Acrothoracica fall into two morphological groups; those of the Trypetesidae and Lithoglyptidae have a well-developed carapace (head shield) that can completely enclose the body and sports fronto-lateral pores, numerous short setae and lattice organs perforated by numerous small, rounded pores and a single, conspicuous terminal pore. The fourth antennular segment has the setae arranged in subterminal and terminal groups. There is a developed thorax with natatory thoracopods and a distinct abdomen and telson. In comparison, the cyprids of the Cryptophialidae exhibit apomorphies in the morphology of the carapace, the antennules and the thorax, mostly in the form of simplifications and reductions. They have a much smaller head shield, leaving parts of the body directly exposed. The shield is conspicuously ornamented by deep pits and hexagonally arranged ridges and bears a few, very long setae but lacks fronto-lateral pores. The lattice organs have numerous elongated pores, but no large, terminal pore. The fourth antennular segment has all the setae clustered in one terminal group. The thorax and thoracopods are rudimentary and not suitable for swimming. These reductions and simplifications in morphology correlate with cryptophialid cyprids being unable to swim. They can only disperse by antennular walking resulting in small, but highly gregarious populations of adults. The variations in antennular morphology and telson structure were traced for the genera of the families Lithoglyptidae and Trypetesidae. The traditional non-cladistic taxonomy in the suborders Pygophora (Cryptophialidae+Lithoglyptidae) and Apygophora (Trypetesidae) was based largely on symplesiomorphies in adult morphology and cannot be upheld. The Lithoglyptidae and Trypetesidae may form a monophylum, but evidence remains scarce. We expect that the use of larval (cyprid) characters will in the future play an important part in more detailed phylogenetic analyses of the Acrothoracica and also shed new light on their reproductive ecology.     

First report of antennular attachment organs in a barnacle nauplius larva

Larvae released from Newmaniverruca albatrossiana were cultured in the laboratory until the cypris stage. The brood size of individuals was low, about 60 larvae per brood. The exact number of instars was not determined. Early instars had the morphology normally seen in lecithotrophic nauplii of thoracican cirripedes. They had uniramous antennules with a few apical setae and biramous antennae and mandibles equipped with natatory setae only. Neither antennae nor mandibles carried any enditic spines or setae and the mouth cone was diminutive. The last nauplius stage obtained in our cultures was typical except in the structure of antennules. The head shield was enlarged but not flexed down, the antennae and mandibles were virtually unchanged from earlier instars, and the ventral thoracic process was well developed but without any external appendages. In contrast, the antennules had the complex shape and segmentation otherwise seen only in cypris larvae, where they are used for bipedal walking on the substratum in search of a settlement site. The similarity included the specialized shape of the first two antennular segments and the specialization of the third as an attachment organ. Nauplii just prior to this last instar had simple, straight antennules but completely lacked setae and instead terminated bluntly in what appears to be an incipient attachment organ. The presence of cypris-like antennules in late nauplii has not previously been recorded in cirripedes. We suggest that this will allow the larvae to attach on the substratum temporarily before they reach the cypris instar and this will increase the chance of settling successfully on their rare substratum (sea urchin spines). The specialization in late N. albatrossiana nauplii will therefore decrease mortality during the larval phase and thus counterbalance the very low breeding potential in this deep-sea species.     

Anatomy of virgin and mature externae of Loxothylacus texanus,parasitic on the dark blue crab Callinectes rathbunae (Crustacea: Cirripedia: Rhizocephala: Sacculinidae)

Rhizocephalan parasites are dioecious organisms, in that one or several dwarf males are implanted into the external part of the female parasite soon after it emerges from the interior of the host animal. The structure of the female externa and its resident males is crucial for understanding both the reproductive biology and the taxonomy of these specialized parasites. We use scanning electron microscopy and histological methods to study the anatomy of juvenile and the mature externae of the rhizocephalan barnacle Loxothylacus texanus parasitizing the blue crab Callinectes rathbunae. We put emphasis on the implantation of males and the histology of the female reproductive organs. In the virgin externae, male cyprids attach around a cuticular hood covering the mantle aperture, which is partially blocked by a plug of cuticle so only trichogon larvae, not cyprids, can access the mantle cavity. This resembles the situation known from Sacculina carcini. The mature externa is characterized by a visceral mass that contains the ovary, paired colleteric glands, a single male receptacle, but paired receptacle ducts. The proximal attachment of the visceral mass is located at some distance from the basal stalk, as is characteristic for the genus Loxothylacus. The internal anatomy of the mature externa of L. texanus is in most features similar to that seen in other species of the Sacculinidae, which comprises the majority of rhizocephalan species. However, the single receptacle creates a situation where the two implanted males cannot be kept separate as in most other rhizocephalans, but pass through spermatogenesis in a common chamber. This may have unknown effects on the reproductive biology such as male–male competition. J. Morphol., 2010. © 2009 Wiley‐Liss, Inc.     

藤壶金星幼虫附着变态机制

藤壶属节肢动物门(Arthropoda)甲壳亚门(Crustacea)蔓足下纲(Cirripedia)围胸总目(Thoracica), 具备特殊的形态结构、生活史和种群生态特征,是最主要的海洋污损生物。其幼虫阶段通常经历6期无节幼体和1期不摄食的金星幼虫,从浮游的金星幼虫附着变态成固着的稚体是藤壶生活史中的一个关键环节。外界化学和生物因子中成体提取物、水溶性信息素、足迹、神经递质、激素、生物膜等均影响藤壶金星幼虫的附着变态;内在因子即金星幼虫的生理状态(能量储量和年龄)决定了其对外界因子的反应程度。概括了近年来藤壶附着变态生理机制和分子机制研究的进展,可为深入了解藤壶金星幼虫附着变态机制提供参考,也为开发新型、高效、环保的防污剂提供理论指导。     

Sexual discrimination in the hermit crabPagurus geminus

Factors concerning sexual discrimination and related sensory receptors were investigated on the hermit crabPagurus geminus. In the breeding season, male crabs of this species were observed to hold receptive females but to reject males. By using this behavior, it was examined whether males on which some operation on the sensory receptors was performed could discriminate between presented male of female crabs. Test males could not discriminate sexes of the presented crabs only when antennules were removed bilaterally. On the other hand, intact test males reacted positively to an empty shell from which “female water” exuded after passing through the chamber containing females. Thus, the factor by which the hermit crab discriminates sexes was indicated to be a water-borne chemical and that the sensory receptor related to sex-discrimination appeared to exist on the antennules. Discussions are made with respect to predicted substances taking part in sexual discrimination of this hermit crab.     

An improved barnacle attachment inhibition assay

The Balanus amphitrite attachment inhibition assay, developed by Rittschof et al. (1992), has been widely used for screening antifouling compounds. One shortcoming of this assay, however, is the low (often < 40%) attachment rate of cyprids, including in the controls that contain seawater only. In this study, trapping of cyprids at the air-water interface was found to be a main contributor to the low attachment rate. Procedures to eliminate the air-water interface were thus introduced. With the improved bioassay, a much higher cyprid attachment rate (>70%) was attained. To further illustrate the usefulness of the improved assay (ie eliminating the air-water interface), the effects of the length of cyprid storage and the effect of a reference biocide, tributyltin chloride, on the survival and attachment rate of the cyprids were examined. The length of cyprid storage was important, with newly molted cyprids, 3- to 9-day old cyprids and 12-day old cyprids having an attachment rate of 43%,>75% and 36%, respectively. The low attachment rate in the newly molted cyprids was due to a high percentage of cyprids that still swam at the end of exposure period, whereas the low attachment rate in the 12-day old cyprids was due to a high mortality rate. The cyprids showed an EC50 of 22 microg l(-1) for attachment inhibition and LC50 of 25 microg l(-1) for mortality. It is concluded that the air-water interface has an important confounding effect on cyprid attachment rate in the conventional B. amphitrite attachment assay. By eliminating the air-water interface more robust quantitative assay results were obtained.     

4 new species of blackflies (Diptera: Simuliidae) from Central Asia, Siberia and the Far East

Four new species of blackflies (Diptera: Simuliidae) are described. Helodon rezidentsii Yankovsky, sp. n., Khabarovsk Territory, differs from all known species of the genus with morphology of eyes consisting only of microommatidii in male; from related species H. kamtshaticus (Rubzov, 1940) with large triangular posteromedial lobes of branches of genital fork in female; with number of rays of primary fan of premandibles (20-22, when in H. kamtshaticus 30-32) in larva. Helodon submulticaulis Yankovsky, sp. n., Transbaikalia, differs from related species H. multicaulis (Popov, 1968) with number of rays of primary fan of mandibles (36-40, when in H. multicaulis 26-28), narrow anterior branches of anal sclerite, number of rows of hooks in posterior attachment organ (88-92, when in H. multicaulis 78-80) in larva; with morphology of respiratory organ, consisting of 5-8 lobes bearing 40-60 tune filaments (in H. multicaulis 3-4 lobes bearing more than 150 filaments) in pupa. Sch. samarkandica Yankovsky, sp. n., Uzbekistan, differs from related species Sch. pseudopusilla Rubzov, 1956 with 3 (not 2 as in Sch. pseudopusilla) hooks in parameres, bifurcated lateral branches of X sternite, long projection of gonostyles in male. Schoenbaueria ivdelensis Yankovsky, sp. n., Middle Ural, differs from related species Sch. rangiferina (Rubzov, 1956) with prolonged gonostyles bearing narrow projection in male; with number of rays of secondary fan of premandibles (20-28, when in Sch. rangiferina 44-48), deep ventral groove of cephalic capsule, number of rows of hooks in posterior attachment organ (80-82, when in Sch. rangiferina 70-72) in larva; with morphology of respiratory organ (very long stems of 2 and 3 pairs of filaments) in pupa.     

Review of naupliar development among Miraciidae (Copepoda,Harpacticoida) with a naupliar description of <Emphasis Type="Italic">Paramphiascella</Emphasis><Emphasis Type="Italic">choi</Emphasis> sp. nov. from Thailand

Both genders of Paramphiascella choi sp. nov. were collected from the green alga Enteromorpha clathrata in Rayong province, Thailand. P. choi shares with other species of the genus: cylindrical body shape, rostrum not bifid, eight-segmented antennules, three-segmented exopodal antenna, and female P5 exopod with five setae. The new species distinguished from other conspecific species by: three-segmented exopodal antenna, inner edge of basis of male P1 and P2 bear a bare ovate-knob each. Enp-1 very elongate, Enp-2 of male P2 transformed into a large, strong, slightly curved and tapering attenuation with two central chitinous ridges, and bearing one medially directed knob close to enp-1. At the base of this knob arise three plumose setae of unequal length. These characters are suggested to be autapomorphies of the new species. Six naupliar stages are obtained and described a key for the identification of stages is provided. Nauplius I has one pair of caudal setae; three-segmented antennules; antenna consists of a coxa, basis, endopod and exopod; mandible has a coxa, basis, endopod and exopod; hindbody bears two caudal setae. Nauplius II develops one aesthetasc on the antennule; antenna has added an arthrite arising from the coxa; mandible has a row of tiny spinulose setae. Nauplius III has added two pairs of caudal setae. Nauplius IV bears bilobed bud of the maxillule armed with two setae and four pairs of caudal setae. Nauplius V bears a multilobed bud of the maxillule with three setae and five pairs of caudal setae. At Nauplius VI, the buds of swimming legs 1 and 2 are added.     

Size and settling behaviour in male and female cypris larvae of the parasitic barnacle Sacculina carcini Thompson (Crustacea: Cirripedia: Rhizocephala)

Twenty-one broods from different externae of Sacculina carcini Thompson were cultured to the cypris stage. The size of the cyprids was measured and the larvae subjected to settling upon unparasitized crabs (Garcinus maenas (L.)) and small juvenile externae. The cyprids occur in two sizes that may appear singly or together in the same brood. Small cyprids are of the female sex that settle upon crabs and are infective, while large cyprids are of the male sex and only settle upon juvenile externae. These results are in agreement with other well-studied rhizocephalans.     

Behavioral and electrophysiological experiments suggest that the antennular outer flagellum is the site of pheromone reception in the male helmet crab Telmessus cheiragonus

Sexually competent females of Telmessus cheiragonus (helmet crab) release two pheromones that elicit grasping and copulation behaviors in males (Kamio et al., 2000, 2002, 2003). Our study aimed to use behavioral and electrophysiological techniques to identify the site of reception of these sex pheromones. In behavioral experiments, either the inner or the outer flagella of the antennules were ablated bilaterally from male crabs, and responses of male crabs to female odor were examined. When the inner flagella were surgically ablated, the sexual response (i.e., grasping and copulation behavior) of male crabs was not significantly changed relative to control animals that had their second antennae ablated. In contrast, the sexual response was significantly reduced when the outer flagella of the antennules were ablated, suggesting that the outer flagellum is the receptor organ that detects the sex pheromones. In electrophysiological experiments, urine, which in females contains the pheromone that elicits grasping behavior by males but does not contain the pheromone eliciting copulation, whose release site is not known, was tested. Female and male urine as well as shrimp extract evoked phasic responses of chemosensory afferents innervating aesthetasc sensilla on the outer flagellum of male crabs. The response of the afferents had significantly higher magnitude and lower threshold when female urine was applied. Thus, behavioral and electrophysiological observations suggest that in male helmet crabs, the outer flagellum of the antennule is the chemosensory organ that detects female sex pheromone.